A control device is known which is constructed from an electronic power module and a separate commercially available switchgear. For the purpose of soft starting and during rotational-speed-controlled operation of a motor, the semiconductor valves of the electronic power module are operated by phase gating control. When the steady-state operation of the motor is reached, the current is taken over by bridging by use of the mechanical switching contact of the switchgear, as is illustrated in a simplified manner in the single-phase electrical circuit according to FIG. 1.
Significantly smaller losses arise compared with electronic motor control units without bridging. In the steady-state ON operating state, the losses of a switchgear amount to approximately {fraction (1/10)} of the losses of power semiconductors. Through suitable driving of switchgear and electronic power part by the control unit of the electronic power part during the transition from rotational-speed-controlled operation to steady-state operation, from steady-state operation to rotational-speed-controlled operation and from steady-state operation to the OFF state, the arc loading on the switchgear contacts can be restricted to a minimum amount and the switching lifetime of the contacts can thus be increased to the region of the mechanical switching lifetime of the switchgear.
The known control devices are constructed in accordance with FIG. 2 with a commercially available switchgear and an electronic power module arranged alongside as separate components which are electrically interconnected.
EP 0 860 145 B1 discloses a control device of the type mentioned above. In the case of this control device, the semiconductors are driven in such a way that only a minimal voltage occurs at the mechanical switching contacts during the very short switch-on and -off phases. This considerably prolongs the electrical lifetime of the switching contacts. The duration of the switch-on and -off phases is very short, i.e. only a few power supply periods, so that likewise only very little power loss arises in the semiconductors. In a motor control unit, however, starting and stopping operations occur which can last from a few seconds to a few minutes. During these control operations, exclusively the semiconductor elements are in use, with the consequence of correspondingly high losses which must be dissipated by means of suitable cooling measures.